There have been known a great variety of catalysts for use in producing unsaturated acids by vapor phase catalytic oxidation of the corresponding unsaturated aldehydes.
U.S. Pat. No. 3,567,773 discloses molybdenum-tungsten-vanadium based catalysts which are known to be especially effective. Also, German Patent No. 2,038,763 discloses molybdenum-vanadium-antimony based catalysts.
These catalysts may produce considerably good results. When these catalysts are to be used for industrial purpose, large amounts of the uniform catalysts are required. However, it is very difficult to produce such catalysts in such large amounts. In addition, further improvement in catalyst life is desirable, and even a slight improvement in yield is advantageous.